The invention relates to a single-phase synchronous motor comprising a two-pole permanent-magnetic rotor and an eddy-current intermediate rotor which is rotationally coupled to the motor shaft and which is arranged to be rotatable between the stator-iron poles and a permanent-magnetic rotor which is mounted for free rotation on the motor shaft.
Such a single-phase synchronous motor is known from DE-GmS 84 00 013. This motor has a cylindrical laminated stator in whose cylindrical interior two stator poles extend, which poles can be magnetised by exciter coils.
In the stator bore a permanent-magnetic cylindrical rotor and a cup-shaped eddy-current intermediate rotor made of copper are arranged on a motor shaft. The eddy-current intermediate rotor is rotationally coupled to the motor shaft and the permanent-magnetic rotor is mounted for free rotation on the motor shaft.
This single-phase synchronous motor thus also operates as a single-phase induction or asynchronous motor; it is preferably used for driving a pump. When this motor is started the permanent-magnetic rotor initially runs up to the synchronous speed and the eddy currents produced in the eddy-current intermediate rotor cause the intermediate rotor to accelerate asynchronously, i.e. substantially more slowly than the permanent-magnetic rotor. One of the advantages of this construction is that the starting torque is substantially larger than that of a true single-phase synchronous motor.
This construction is intricate and results in bearing problems because the cup-shaped intermediate rotor is supported at one end only. Manufacturing such a cup-shaped intermediate rotor is expensive and its bearing construction is critical.